660 BACTERIUM 



absence of indole production in peptone water, and the production of hydrogen 

 sulphide have served as additional differential criteria ; and other tests, such as the 

 final pH attained in a dextrose-containing medium, the nature and amount of the 

 gases evolved, or the production of some particular fermentation product, have been 

 employed as aids to differentiation within particular sub-groups. 



It soon became clear that the presence or absence of the power to ferment lactose, 

 originally noted as differentiating Bad. coli from Salm. typhi, corresponded to a 

 fundamental line of cleavage within this group. The lactose fermenters were, for 

 the most part, found to be normal inhabitants of the intestinal tract of man or the 

 higher animals or to exist on various plants or in the soil. They were active 

 fermenters of many carbohydrates, including polysaccharides ; they tended to clot 

 milk, as well as acidify it ; they frequently formed indole ; and they tended to 

 reduce various dyes (Dunbar 1892, Rothberger 1898). The non-lactose fermenters 

 tended, as a class, to comprise the pathogenic species, producing intestinal infections 

 in man and animals ; and the range of their fermentative activity tended to be less 

 extensive than that of the lactose fermenters, though most species attack a consider- 

 able number of substrates. 



This early division of the genus into two broad sub-groups on the basis of the 

 lactose fermentation has stood the test of time, although there are a few species or 

 types for which some intermediate position must be found. As already noted, 

 the genera Salmonella and Shigella have been created for the more important 

 of the non-lactose fermenters. These organisms will be considered in separate 

 chapters. In the remainder of the present chapter we shall devote ourselves to 

 a consideration of the lactose-fermenting organisms, together with the indeter- 

 minate group of paracolon bacilli, which appear to be more nearly related to the 

 coliform bacilli than to the Salmonella or Shigella organisms. Though no clear 

 line of demarcation can be laid down between the coliform bacilli and the Fried- 

 lander group of bacilli, it will be convenient to describe them separately. 



The Coli-Aerogenes Group 



Biochemical Differentiation. — In his original description of Bact. coli, Escherich 

 noted the occurrence of two types, one of which, Bact. coli, formed relatively long 

 rods, was motile and clotted milk slowly, while the other, 5«c/. lactis aerogenes,ioTmed 

 shorter, plumper rods, was non-motile, and clotted milk more actively. Kruse 

 (1894) emphasized the heterogeneity of the group covered by the term " B. coli " as 

 usually employed, pointing out that it included a variety of related species, widely 

 distributed as intestinal parasites and in water and soil. The use of a relatively 

 small series of fermentation tests, including especially dextrose, lactose, sucrose, 

 starch, inulin, action on litmus milk, and indole formation, resulted in the recognition 

 of certain primary divisions within this group (Rej&k 1896, Grimbert and Legros 

 1900, Durham 1901, Jordan 1903). One of the groups so defined fermented poly- 

 saccharides, such as starch and inulin, and usually failed to form indole ; this corre- 

 sponded with the Bact. lactis aerogenes type of Escherich and became established as a 

 separate species, the lactis being usually omitted from the name. The second and 

 third groups differed from Bact. aerogenes in failing to ferment starch and inulin, and 

 in forming indole in peptone water. They differed from each other in regard to 

 their action on saccharose. One, corresponding to the existing strains of Escherich's 

 Bact. coli commune, failed to ferment this sugar ; the other fermented it, and 

 Durham (1901), who found it to occur more frequently than the saccharose-negative 



